Exam class: write details of points with \totalpoints

Question

I use the exam class to produce exams for students, and I want to make a kind of questions, which puts in front the total points, but given detailed. I give an example:

\documentclass[addpoints]{exam}

\pointformat{}
\qformat{\textbf{Question \thequestion}. [Points of question \thequestion: \totalpoints]\hfill}

\begin{document}
\begin{questions}
\question
\begin{parts}
\part[3]
\part[2]
\end{parts}
\end{questions}
\end{document}

which gives this output:

I want to obtain, as output, the detailed points of the question, so precisely something like:

[Points of question 1: 3+2=5 points]

How can I obtain this ?

Answer 1

The key was figuring out the \@doitem was actually processing the argument. While it is defined globally, I only want to modify/replace it for parts.

The \expandafters are to expand \else and \fi first.

\documentclass[addpoints]{exam}

\pointformat{}
\qformat{\textbf{Question \thequestion}. [Points of question \thequestion: \details\totalpoints]\hfill}

\makeatletter
\newcommand{\details}{\@ifundefined{details@\roman{question}}{}{\csname details@\roman{question}\endcsname =}}

\newcommand{\@details}{}% reserve global name

\let\normal@parts=\parts
\let\endnormal@parts=\endparts
\renewenvironment{parts}% no arguments?
 {\xdef\@details{\@empty}% initialize
  \normal@parts
  \let\normal@part=\@doitem
  \let\@doitem=\my@part
 }{%
  \endnormal@parts
  \ifx\@empty\@details
  \else
    \immediate\write\@auxout{\string\gdef\string\details@\roman{question}{\@details}}
  \fi
 }

\newcommand{\my@part}[1][\@empty]{%
  \ifx\@empty#1\relax
    \expandafter\normal@part
  \else
    \ifx\@empty\@details
      \xdef\@details{#1}%
    \else
      \xdef\@details{\@details +#1}%
    \fi
    \expandafter\normal@part\expandafter[\expandafter#1\expandafter]%
  \fi
}%
\makeatother  

\begin{document}

\begin{questions}
\question[5]
\question
\begin{parts}
\part[3]
\part[2]
\end{parts}
\end{questions}
\end{document}